Return list of nodes inside material?
You don't need their names in a list. You can iterate through Material.node_tree.nodes collection which allows to test if the node type is Image Texture and set the position attribute in one go.

Operator example based on Templates > Python > Operator Node:

import bpy

class NodeOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "node.simple_operator"
    bl_label = "Simple Node Operator"

    @classmethod
    def poll(cls, context):
        space = context.space_data
        return space.type == 'NODE_EDITOR'

    def execute(self, context):
        tree = context.space_data.node_tree

        # set the location if node type is
        for n in tree.nodes:
            if n.type == "TEX_IMAGE":
                print (n.location.x)
                n.location.x += 50

        # filter by type
        image_texture_nodes = [n for n in tree.nodes if n.type == "TEX_IMAGE"]

        # get the names
        image_texture_names =  [n.name for n in image_texture_nodes]

        # check the console
        print (image_texture_nodes)
        print (image_texture_names)

        return {'FINISHED'}


def register():
    bpy.utils.register_class(NodeOperator)


def unregister():
    bpy.utils.unregister_class(NodeOperator)


if __name__ == "__main__":
    register()